1. Field of the Invention
This invention is generally related to instrumentation used for analyzing microstructures in minerals. More particularly, the invention allows an inexpensive and effective means for observing fluid inclusions, microcracks, and other structures within crystal and other mineral samples.
2. Description of the Prior Art
Morphometric analysis of objects or structures within translucent materials is of considerable interest to geologists and materials scientists. The orientation of planar and linear arrays of fluid inclusions with respect to crystallographic directions in the host mineral is the most important characteristic for determining the temporal classification of inclusions. Knowledge of the orientation of inclusion planes relative to macrostructural features in the host rock is necessary to understand the timing of various fluid events with respect to the tectonic and deformational history of the rock. In addition, the size and shape of individual fluid inclusions and their contained phases must be known for many important applications.
At the present time, there is no inexpensive, straight-forward method for analyzing microstructures such as fluid inclusions and microcracks within mineral samples.
In standard petrographic techniques, the sample is observed in only two dimensions. Three-dimensional properties can be inferred from the two dimensional images using sophisticated mathematical models. De Hoff, J. of Microscopy, 131:259-263 (1982), disclose a technique for quantitative serial sectioning analysis for characterizing three-dimensional microstructures from two-dimensional image information.
Recently, Petford et al., Am. Mineralogist, 77:529-533 (1992) disclosed three-dimensional imaging of fluid inclusions using confocal scanning laser microscopy. In operation, confocal three-dimensional images are achieved by compiling serial sections through the sample with the aid of image analysis software. Despite the advantages, this technique has the disadvantages of requiring a very high instrumentation cost, requiring very complex equipment and software which is not easily understood by researchers, and requiring correction for the refractive index of the host material. Furthermore, materials that are transparent to white light may not be easily observed with laser light.
The spindle stage is a simple and powerful tool used to measure the optical constants of anisotropic crystals by immersion techniques. The attributes and functions of the spindle stage are discussed in detail in Bloss, Am. Mineralogist, 63:433-447 (1973) and Bloss, The Spindle Stage: Principles and Practice, Cambridge University Press, New York, 1981 , 340 pages. For optical studies, it is expedient to use crystals that are typically less than one millimeter (1 mm) in diameter. Therefore, conventional spindle stages are equipped with a shallow immersion cell that is sufficient to completely bathe a tiny crystal in oil. Modified spindle stages are described in Roy, Am. Mineralogist, 50:1441-1449 (1965), Jones, Am. Mineralogist, 53:1399-1403 (1968), and Grattan-Bellew, Am. K. of Science, 274:829-830 (1974). Each of these modified spindle stages are utilized for analyzing very small crystal samples, and each includes the use of an immersion cell with an open side. Oil is retained in the immersion cell via the surface tension with the top and bottom plates, and by adding blotting paper between the plates. Since only small crystal samples are used, oil leakage from the immersion cell is minimal.
Prior to this invention, the spindle stage had not been used for analyzing structures such as fluid inclusions and microcracks in larger mineral samples.